Abstract: A method for measuring the surface temperature of a wafer substrate and a heat-treating apparatus are both used in a semiconductor device-manufacturing process wherein a reference light including infrared rays is intermittently emitted toward a wafer substrate. Infrared rays of a plurality of different types whose wavelengths are shorter than 1 .mu.m and differ from each other are selectively detected from the reference light emitted toward the wafer substrate, the reference light reflected by the wafer substrate, and the light radiated from the wafer substrate itself. On the basis of the detection performed with respect to the emitted reference light, the reflected reference light and the radiated infrared rays, the surface temperature of the wafer substrate is calculated. A substrate-heating element is controlled on the basis of the calculated temperature.